The present invention relates a stainless steel for case hardening with nitrogen.
Case-hardened steels generally have a low content of alloy and contain, for instance, 0.15 to 0.20 wt. % carbon. By surface carburizing to 0.5 to 1.0 wt. % C, followed by hardening, structural parts having a tough core and a hard, wear-resistant surface layer which is under internal compressive stresses are obtained. This condition of internal stressing leads to an increase in the static and cyclical strength of structural parts such as gears and ball bearings, for example.
Stainless structural parts are desired in certain fields of use. Thus, for instance, ball bearings for airplanes are made of case-hardened stainless steels such as, for instance, X 105 CrMo 17 (AISI 440 C). In order to increase the static and cyclic strength of stainless structural parts, a stainless case-hardened steel which contains the following alloy components (wt. %)
______________________________________ C 0.05-0.1 Mn .ltoreq.1.5 Si .ltoreq.1 Cr 11-15 Mo 1-3 Ni 1.5-3.5 Co 3-8 V 0.1-1 N .ltoreq.0.04 ______________________________________
has been developed (see EP 0 411 931 A1).
Chromium and molybdenum impart resistance to rusting to this steel. Manganese, nickel and cobalt serve, in known manner, to suppress .delta.-ferrite in the core, and vanadium promotes the resistance to tempering. As a result of the high alloy content, the mixed-crystal hardness in the core increases, so that a smaller content of carbon is necessary for establishing the core hardness than in the case of low-alloy case-hardened steels. Nitrogen is preferably limited to .ltoreq.0.002 wt. %. Structural parts of this steel are case hardened with carbon.
Federal Republic of Germany 40 33 706 C2 describes a method for the heat treatment of stainless martensitic steels in which the carbonizing is replaced by nitriding. Nitrogen, like carbon, is capable of increasing the surface hardness, but it promotes the chemical resistance of the martensite while carbon decreases it. Case hardening with nitrogen therefore gives the highest resistance to corrosion of the surface layer if it is practically free of carbon.